The present invention relates to an expandable drill bit to be used in oil drilling operations for the purpose of cleaning solidified cement from within the perforated casing of a previously drilled well which has been sealed with the cement.
A typical bore resulting from drilling for oil is fitted with a well casing or liner made of steel. At some point within the casing a permanent interior collar, known as a restriction, is fitted to restrict the inner diameter so that an additional "string" or pipelike member, through which the oil will flow, can be inserted and the flow of oil directed into the string. The restriction typically has an internal diameter approximately one-half of the diameter of the inner diameter of the casing in which it is used. The oil producing portion of the well is located below this restriction. The walls of the well casing are perforated using a perforating "gun" to allow the flow of oil into the casing from an area surrounding the casing where oil is retrievable. At times it may be necessary to seal off a perforated area to prevent intrusion of undesirable material such as water or to realign the perforated area with the oil bearing area. A common practice is to seal or "squeeze" a perforated casing by filling a portion of it completely with cement which hardens in place, eliminating perforations in undesired areas. However, after sealing in this manner, it is generally necessary to ream or clean out all or part of the cemented casing which is, by necessity, located below the restriction, and the restriction prevents such reaming with the use of a cutting or drilling bit with a fixed diameter larger than that of the restriction opening. Sealing with solid cement and then reaming out the desired portions of the well casing by using an expandable drill bit in accordance with this invention has many advantages over selectively sealing only the casing walls in the perforated area including:
(1) A higher quality can be retained in the cement by preventing the introduction of contaminants. PA0 (2) The cost of the operation can be lower since no pilot hole is required. PA0 (3) The full size of the internal bore of the casing remains after reaming, allowing the largest perforating guns to be used when reperforating the casing. PA0 (4) The interior surface of the casing remains relatively smooth and consistent, eliminating the possibility of cement boulders falling in and interfering with or sticking guns during the subsequent perforating operation.
An ordinary drilling bit, which usually has three rotating cones, provides the greatest drilling speed, but will not pass through a restriction and therefore cannot be used to clean a casing to its full inner diameter. Prior art devices known as "underreamers" have previously been used to accomplish the task of cleaning out a casing. A typical underreamer consists of a flat faced pilot bit or guide mounted to a cylindrical head no larger in diameter than the restriction through which it must pass. Recessed into the side of the cylinder are two pivotably mounted cutting heads with cutting surfaces capable of pivoting outward to an expanded diameter to cut a bore diameter equal to the size of the casing. Because these cutters must be retracted into the cylindrical cutting head when not in use, the cross sectional area of the cutting head is necessarily decreased by any increase in size or number of the cutters. Because of this decrease in cross sectional area, the head is necessarily weakened, and so is generally limited to two cutters. Also, where the cutters must be retracted into recesses within the sides of the cylindrical body, any bending or deformation of these cutting surfaces may prevent retraction of the cutters into the cylinder body. An inability to retract the cutters may, in turn, prevent the tool from passing through a restriction to remove it from a well and may require abandonment of the tool string and/or the well itself. Because of the significant expense of well drilling and the cost of equipment involved, it is obviously desirable to minimize the possibility of any problem which may result in abandonment of the well or any equipment. Also, the flat face of the typical pilot bit or guide does not readily provide a guide for the tool when either (1) inserting the tool into a well and through square shouldered restrictions, or (2) cleaning hardened cement from a well.